Radley Balko points out that Biden’s voting record on drug issues in particular and civil liberties in general is quite bad. (Edit: As a couple of commentators have noted, Biden’s overall civil liberties record isn’t as bad as Balko is making it out to be).
FWIW I think on average too much attention gets paid to VP picks (this rule contains an exception for nominating one of Satan’s actual minions). Still there are a lot of things about the Biden pick that are troublesome. Yglesias notes that Biden’s Iraq vote seems to have been based on the rather strange theory that an Iraq war skeptic could influence the process in a positive way by supporting the Bush administration’s push for maximum latitude in going to war.
Further thoughts: What really bugs me about the Biden pick is that he voted for the war, and picking him inevitably blurs the message of what a gigantic mistake that was. It also reinforces the myth that it was “political suicide” to vote against the war in 2002. In fact more than half of the Democratic members of Congress voted against the war, including a solid minority of senators, and of course far from being political suicide, there’s no real doubt Clinton would be the nominee if she had simply done the right thing at the time. As Scott pointed out yesterday, the Beltway wisdom on Iraq remains so twisted that having voted for the war makes Biden more acceptable in the eyes of the Villagers.